It is generally known that alkaline storage battery separators use a non-woven fabric sheet having a polyolefin based fiber. Various kinds of olefin based separators which have chemical resistant properties can undergo severe cell reaction when the temperature of the battery electrolyte is increased to about 80 .degree. C. In addition, polyolefin based fibers such as polypropylene fiber and polyethylene fiber which are alkali and oxidation resistant have fatal faults in that the polyolefin based fibers have poor affinity with battery electrolyte and are poor at holding the battery electrolyte.
In order to solve the above mentioned problems, polyolefin based non-woven fabric sheets which are treated with a surface active agent are disclosed in Japanese unexamined laid open patent applications (Tokkai-sho) No. 147956/1983, 194255/1983 and 39362/1986. A non-woven fabric sheet separator using vinylone is disclosed in Japanese patent applications (Tokko-sho) No. 26942/1981 and a non-woven fabric sheet separator using an ethylene-vinyl alcohol copolymer fiber is disclosed in Japanese unexamined laid open patent applications (Tokkai-sho) No. 34849/1983. A non-woven fabric sheet separator using a sulfonated polyethylene fiber or polypropylene is disclosed in Japanese unexamined laid open patent application (Tokkai-sho) No. 175256/1983, (Tokkai-hei) Nos. 57568/1989, and 132043/1989, and in U.S. Pat. No. 5,100,723 (EPC patent publication No. 316916A2/1989).
However, according to the above references, the hydrophilic property of a non-woven fabric sheet separator which is treated with a surface active agent was reduced by the elution or deterioration or decomposition of the surface active agent as a result of repeated charging and discharging for a long period. A non-woven fabric sheet separator using vinylon, or an ethylene-vinyl alcohol copolymer fiber had some problems in that the life of a storage battery was shortened due to deterioration and developed a short-circuit which was caused by extended polymer deterioration by an oxidation reaction which a strong alkaline battery electrolyte.
According to the sulfonated storage battery separators mentioned above, however, .alpha.-polyolefin was subjected to sulfonation with concentrated sulphuric acid at higher than 100.degree. C. or with highly concentrated fuming sulphuric acid or the like. This means that hydrogen substitution in a tertiary carbon is involved. The efficiency of this reaction was inferior. In addition, the introduction of a desired amount of sulfo groups was difficult and the sulfonated storage battery separators above mentioned posed many problems in the process of manufacturing causing considerable damage of the fiber. Further, according to the above reference, the non-woven fabric sheet separator have high rigidity. Therefore, it is not good workability of the assembly storage battery operation.